1. Field of the Invention
The invention relates to a photothermographic material, and more particularly, to a photothermographic material revealing a soft gradation and stable in output image and an image forming method thereof.
2. Description of the Related Art
In the medical imaging field and the graphic arts field, there has been, in recent years, a strong desire for a dry photographic process from the viewpoints of environmental conservation and space-saving. Further the development of digitization has resulted in the rapid development of systems in which image information is captured and stored into a computer. If necessary, the image information is processed by the computer which outputs the image information through communication to a needed location; at the site, the image information is further output onto a photosensitive material using a laser-image setter or a laser-imager, followed by development thereof to form an image on the photosensitive material. It is required that the photosensitive material be able to record an image under exposure to a laser with a high illuminance and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using a pigment and a dye such as an ink-jet printer or electrophotography have been distributed as a general image forming system using such a digital recording imaging material, images in the digital recording imaging material obtained by such a general image forming system are insufficient in terms of image qualities required for medical images. To facilitate diagnosis, image qualities such as sharpness, granularity, gradation, tone and high recording speed (sensitivity) are required. However, digital recording imaging material has not reached a level at which it can replace medical silver salt film processed by conventional wet development.
A thermographic system using an organic silver salt is described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075; and D. Klosterboer, “Thermally Processed Silver Systems”; J. Sturge, V. Walworth and A. Shepp (coedited), “Imaging Processes and Materials” Neblette 8th Ed., Chapter 9, pp. 279-291, 1989, the disclosure of which is incorporated by reference herein. Generally, a photothermographic material, in particular, comprises an image forming layer in which a photosensitive silver halide, a reducing agent, a reducible silver salt (for example, an organic silver salt) and if necessary, a toner controlling a color tone of diveloped silver are dispersed in a binder matrix.
A photothermographic material forms a black-toned silver image by heating a photothermographic material to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by a catalytic action of a latent image generated on the silver halide by exposure. As a result, a black-toned silver image is formed in an exposed region. A photothermographic material is disclosed in many literatures including for examples U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924; Fuji medical dry imager FM-DP L is an example of a practical medical image forming system that has been marketed.
Since such a thermographic system using an organic silver salt has no fixing step and the photosensitive material contains all chemicals necessary for image forming, there has been an intrinsic problem in raw preservability, that is “increase in fog”, that an unexposed portion is blackened during storage from manufacture of a photosensitive material till the material is actually put into use, and another internal problem of “print-out”. In “print-out” an image after thermal development is exposed to illumination of weak light such as room light. An unexposed portion is gradually blackened.
As means for reducing fogging and print-out, a method in which silver iodide formed through conversion of an organic silver salt is disclosed in U.S. Pat. No. 6,143,488, EP No. 0922995, the disclosures of which are incorporated herein by reference. As to other photosensitive materials using silver iodide, disclosures thereof are given in WO (Laid-Open) Nos. 97-48014 and 97-48015; U.S. Pat. No. 6,165,705; Japanese Patent Application Laid-Open (JP-A) No. 8-297345; and Japanese Patent No. 2785129.
While a silver halide emulsion with a high silver iodide content is generally low in sensitivity, it has been found that a problem of a low sensitivity is solved by writing with a high illuminance light source such as a laser beam and image recording is enabled with less energy. In recent years, development has been made particularly on a light source module with an SHG (a second harmonic generator) and a laser diode integrated into a single piece whereby a laser output apparatus in a short wavelength region has come into the limelight. A blue laser diode enables high definition image recording and makes it possible to obtain an increase in recording density and a stable output over a long lifetime, which results in expectation of an expanded demand in the future for silver halide emulsions with a high silver iodide content: A silver halide emulsion with a high silver iodide content is photosensitive to wavelengths of the blue laser diode without adding a sensitizing dye, which is a feature that other silver halides lack.
However the use of a silver halide with a high silver iodide composition has created a problem in that gradation is harder.
Thus there is a need in the art for an improved photothermographic material including a silver halide emulsion rich in silver iodide content; such an improved material could be effectively used to fulfill the demand for stable photothermographic materials capable of producing high-quality images.